1. Field of the Invention
The present invention generally relates to an image data compressing technique and, more particularly, to an image processing system which compresses and expands image data by using a sub-band encoding method.
The image processing system related to the present invention may be used in a digital copy machine, a facsimile machine, a digital printer, a digital camera or a digital video camera, and also may be used in an image recording system such as a CD-ROM drive or a floppy disc drive.
2. Description of the Related Art
A sub-band encoding method such as the discrete cosine transform (DCT) or the Harr Wavelet transform is considered a method for effectively compressing a continuous tone image. Additionally, Japanese Laid-Open Patent Application No.2-305272 discloses another method for encoding image data by separating an image area into a character area and a halftone area so as to encode these areas by an encoding method appropriate for each of the areas.
Such a method for compressing image data using the sub-band transform such as the DCT or the Harr Wavelet transform can effectively compress a continuous tone image. However, there is a problem in that a compression rate is low when a complete binary image is compressed.
Additionally, in a digital copy machine, even if an original image is a complete binary image, image data obtained by scanning such a complete binary image may become incomplete binary image data due to fluctuation in a scanning operation. Thus, there may be a problem in compressing such incomplete binary data by an entropy-encoding method due to fluctuation in a scanned image.
As a method for rotating or sorting images in a copy machine, a block truncation encoding (BTC) which is one of fixed length encoding methods is popular. However, there is a problem in that a compression rate for an entropy encoding is low as compared to that of a sub-band transform method, and a calculation is complex.
In an image forming apparatus such as a copy machine or a printer, image data obtained by a scanner is subjected to gamma correction or a filtering process so as to adjust image quality. The thus-processed image data is stored in a memory, and then the image data is sent to a printing unit.
Generally, such image data is subjected to a data compression in order to reduce a capacity of the memory that stores the processed image data. Generally, in a data compressing method, image data is transformed into frequency components by using an orthogonal transformation such as the discrete cosine transform (DCT), and the quantized image data is subjected to an entropy encoding. Dispersion of a high-frequency factor in the frequency transformation factors varies in response to a magnitude of change in intensity of the image. Thus, the image quality is improved when a quantizing method is changed in response to a type of an area to be processed.
Japanese Laid-Open Patent Application No.7-74959 discloses a technique in which a quantization table is changed based on a transform factor obtained by an orthogonal transformation of an original image by each individual block so that the image quality matches the contents of the image data and a compression rate is improved.
When an image is printed by a copy machine, a character image and a line image can be well recognized by rendering the intensity slope of a contour of the characters or the lines to be steep. On the other hand, when an image having a gentle intensity slope such as a photograph is printed, a random change in the intensity having a small amplitude is sensed as a noise. Thus, it is preferred for such a photographic image to reduce the intensity slope of an output image. Particularly, in a mesh point photographic image, a better image quality can be obtained by reducing the intensity slope even for an area having a steep intensity slope.
Accordingly, an edge area corresponding to a character image or a line image is separated from a mesh point image and a gentle slope area of a photographic image so that the edge area is subjected to a differential filtering process whereas the photographic image is subjected to a smoothing filtering process. Additionally, when an image data compression is performed, another separation of image areas is performed in response to degrees of the intensity slope in edge areas.
As mentioned above, in the conventional technique, two separation processes are performed on the same image data and the filtering process is performed separately from the quantizing process. Thus, there is a problem in that a process time is increased and a hardware cost is increased. Additionally, there is a disadvantage in the technique disclosed in the above-mentioned patent document in that a compression rate is not minimized since a result of the area separation must be also stored as the compressed data.
An image data compression technique is generally used in the image data processing field so as to reduce a capacity of a memory for storing image data or reduce a time for transmitting image data. There are various image data compressing methods depending on the processing modes of image data. When image data is printed, a rotation of the image may be requested. In order to rotate the image at a high speed, a fixed length compression is used.
Additionally, when image data is exchanged between systems having different resolutions or gradation characteristics, a compressing method using a layered data structure is desired so as to select transmission data corresponding to an image quality of an image outputting system. Especially, when image data is transmitted to a display apparatus, a progressive transmission method is required. In the progressive transmission method, image data of an object such as an icon can be transmitted prior to sending the image data. Thus, data compression is performed in response to the level of layers.
Additionally, when a trial printing is performed for checking a layout while reducing toner consumption in an image printing apparatus, a data compressing method is required by which a feature of the image is maintained but image quality is not reduced.
Japanese Laid-Open Patent Application No.1-135265 discloses a data compressing method in which an original image is divided into a plurality of blocks, and each block is divided into image data which is orthogonal-transformed and other data so that a representative image of the image file can be effectively regenerated.
Generally, an image comprises an image area and an edge area. In the image area, a gradation of the image gradually changes, such as in a photograph or a graphic image. In the edge area, a gradation sharply changes in an area of an edge of the image and an area adjacent to the edge, such as in a character image or a line image. When the visual sense of human beings is considered, gradation is important in the image area whereas resolution is important in the edge area.
In the conventional technique disclosed in the above-mentioned patent document, a sampling is performed on the original image, and the sampled image data is subjected to the discrete cosine transform (DCT). The same transform factor which is obtained from a quantization table is used for all areas. In such a case, a length of data is fixed since a single quantization table is used. However, there is a drawback in that the quantized image data does not accurately represent the feature of the image while a large amount of data is used since a single quantization is used. Additionally, there is a disadvantage in that only two levels of image data can be selected.
It is a general object of the present invention to provide an improved and useful image processing system in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an image processing system which compresses an image including both a binary image and a natural image by a sub-band transform method with a high compression rate.
Another object of the present invention is to provide an image processing system which facilitates processing and editing of image data by using a sub-band transform method or a fixed length encoding method at a low cost.
A further object of the present invention is to provide an image processing system which can represent a feature of an image while a reduced amount of data is used, and which can produce image data including a plurality of levels of image quality.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an image processing system comprising:
a buffer unit extracting nxc3x97m pixel matrix, block data from image data, where n and m are integers;
a sub-band transform unit transforming the nxc3x97m pixel matrix block data by a sub-band transform method so as to obtain a transform factor having a plurality of frequency components; and
a quantizing unit quantizing the transform factor by a fixed-length quantizing method by deleting a predetermined number of lower order bits of each of the frequency components.
According to the above-mentioned invention, since the high-frequency component of the sub-band transform factor is quantized by deleting the lower order bits in the fixed-length quantizing method, various subsequent processes such as editing of the image or a rotation of the image can be easily performed with a reduced amount of compressed data.
Additionally, there is provided according to another aspect of the present invention an image processing system comprising:
a buffer unit extracting nxc3x97m pixel matrix block data from image data, where n and m are integers;
a sub-band transform unit transforming the nxc3x97m pixel matrix block data by a sub-band transform method so as to obtain a transform factor having a plurality of frequency components including a low-frequency component and a high-frequency component;
an area discriminating unit discriminating a type of an image area corresponding to the nxc3x97m pixel matrix block data being processed so that the image area is determined as one of an edge area and a non-edge area, a discrimination being made based on whether or not an absolute value of each of the components of the transform factor exceeds a threshold value; and
a quantizing unit quantizing the transform factor by a fixed-length quantizing method by deleting a predetermined number of lower order bits of each of the frequency components, the number of deleted lower order bits of each of the frequency components of the transform factor being changed in accordance with a type of image area being processed so that image data including the transform factor and flag information indicating a type of image area has a predetermined fixed length.
According to this invention, a number of the lower order bits of each of the frequency components of the transform factor is deleted in response to a type of the image, and the transform factor is quantized by a fixed-length quantizing method together with the flag information. Thus, the image data can be efficiently compressed with a high quality irrespective of whether the image data corresponds to an edge area or a non-edge area. Additionally, the original image data can be easily restored on a decoder side based on the flag information.
In the above-mentioned invention, the quantizing unit may delete lower order bits of each of the low-frequency component and the high-frequency component so that a number of deleted lower order bits for the edge area is greater than a number of deleted lower order bits for the non-edge area. Accordingly, data corresponding to both the edge area in which a gradation is important and the non edge area in which recognition of an edge is important can be efficiently compressed while a high image quality is maintained. Additionally, the original image data can be easily restored on a decoder side based on the flag information.
Additionally, the quantizing unit may quantize the high-frequency component by a vector quantizing method. Further, the quantizing unit may embed the flag information into the transform factor.
Additionally, the quantizing unit may change bit data representing the transform factor so that the flag information is represented by a part of the data bits representing the transform factor. The quantizing unit may change the transform factor so that correlation between the transform factors of different types is increased. The quantizing unit may change a bit arrangement of the transform factor so that correlation between the transform factor corresponding to the edge area and the transform factor corresponding to the non-edge area is increased.
Additionally, there is provided according to another aspect of the present invention an image processing system comprising:
a buffer unit extracting nxc3x97m pixel matrix block data from image data, where n and m are integers;
a binarizing unit transforming the nxc3x97m pixel matrix block data into binary data represented by a maximum value and a minimum value;
a differential data calculating unit calculating differential data which is a difference between a value of each pixel in the nxc3x97m pixel matrix block data and one of the maximum value and the minimum value of the binary data;
a sub-band transform unit transforming the differential data by a sub-band transform method so as to obtain a transform factor having a plurality of frequency components; and
an encoding unit encoding the binary data and the sub-band transform factor so as to obtain a code representing the image data.
According to the above-mentioned invention, a continuous tone image data can be efficiently compressed by a sub-band transform since the original image data is represented by using the binary data and the sub-band transform factor which are encoded by a sub-band transform. Accordingly, an image including a binary image and a continuous tone image can be processed by a single method irrespective of types of the image.
In the above-mentioned invention, the encoding unit may delete lower order bits of the sub-band transform factor so that the code has a predetermined fixed length. Accordingly, the image data including the binary image data and the continuous tone image data can be compressed with a high compression rate while a high image quality is maintained. The compression rate is higher than that of the block truncation encoding method.
Additionally, the encoding unit may delete a greater number of lower order bits from the high-frequency component than the low-frequency component when both the maximum value and the minimum value exist in the binary data of the same block data. Accordingly, a change in an average intensity in a block having a sharp gradation change can be prevented, resulting in a prevention of deterioration of the image quality.
Further, the encoding unit may quantize the high-frequency component of the sub-band transform factor by a vector quantizing method.
Additionally, there is provided according to another aspect of the present invention an image processing system comprising:
a dividing unit dividing image data into a plurality of nxc3x97m pixel matrix block data, where n and m are integers;
a transform unit transforming each pixel in the nxc3x97m pixel matrix block data by a frequency transform method so as to produce a transform factor including a high-frequency component and a low-frequency component;
an image area discriminating unit for determining whether the block being processed corresponds to an edge area or a non-edge area based on the transform factor output from the transform unit;
a quantizing unit quantizing the transform factor for the edge area and the transform factor for the non-edge area by different methods; and
an encoding unit encoding an output of the quantizing unit by an entropy encoding method,
wherein a total of a number of bits of the high-frequency component and a number of bits of the low-frequency is the same regardless of types of the edge area or the non-edge area, and a number of bits of the high-frequency component for the edge area is the same as a number of bits of the low-frequency component of the non-edge area.
According to the above-mentioned embodiment, image data representing a feature of the original image can be produced while an amount of data is reduced. Additionally, image data corresponding to a plurality of image quality levels can be produced.
In the above-mentioned invention, the encoding unit may also encode error data generated by the quantizing unit. Accordingly, the restored image data can almost completely match the original data.
Additionally, an encoding of the image for the edge area may be performed by using only the high-frequency component, and an encoding of the image for the non-edge area is performed by using only the low-frequency component. According to this invention, the image quality of the restored image may be low, but a feature of the original image can be sufficiently maintained.
Further, in the above-mentioned invention, every other block data may be used for restoring an original image. According to this invention, the image quality of the restored image may be low, but a feature of the original image can be sufficiently maintained while an amount of data is reduced. Thus, a reduced-size image can be easily obtained.
Other objects, features and advantages of the present invention will become more apparent from the following detailed descriptions when read in conjunction with the accompanying drawings.